What distinguishes viruses from other microorganisms?

Viruses are distinguished from other microorganisms primarily by their nature as the smallest, non-cellular entities devoid of organelles. Unlike bacteria, fungi, and other microorganisms, which are cellular and can carry out metabolic processes independently, viruses lack the machinery necessary for self-reproduction and metabolism. They exist as simple structures composed of genetic material (either DNA or RNA) surrounded by a protein coat and sometimes a lipid envelope.

This non-cellular characteristic means that viruses cannot survive or replicate outside of a host cell, relying on the biological processes of host organisms to multiply and propagate. This distinction places them in their own category, separate from cellular life forms, which typically possess complex organization and cellular structures essential for life functions.